It seems that the latest FlightGear source will not compile against certain header files in the latest SimGear. I obtained the source on 17 November 2020 with "git clone git@github.com:FlightGear/simgear.git" and "git clone git@github.com:FlightGear/flightgear.git". According to "git log", my FlightGear is at commit 22de9d30b518646894ac190cc6b04371daa6d5c2 and my SimGear is at commit 02b20462e183e55e4e2ee7d183a347358ec10b72.
For example, in flightgear/include/simgear/std/type_traits.hxx I had to comment out things like #ifndef HAVE_STD_REMOVE_CVREF_T. Likewise in integer_sequence.hxx. (I'm on Ubuntu 18, and g++ 7.5.0.)
I "fixed" these and other problems, and plowed ahead until I finally got stopped (cmake reporting 45% done) at a static_assert in Ghost (while compiling flightgear/src/GUI/FileDialog.cxx).
Is there another version of SimGear I could checkout, that would not cause these problems when I try to compile FlightGear?